A sonic flow meter is an apparatus for measuring the flow rate of liquids through a channel. Typically, the meter includes at least two transducers, one transducer positioned upstream from the other transducer. The flow meter determines the flow rate of the fluid by transmitting pulses of sonic energy between the two transducers and by measuring the transit time of pulses through the fluid. When the fluid in the channel is stationary, the transit times of the pulses are the same, regardless of which direction the pulses travel through the fluid. However, when the fluid is moving, the motion of the fluid decreases the transit time of the pulses transmitted in the downstream direction and it increases the transit time of pulses transmitted in the upstream direction. The increase or decrease in the transit time is proportional to the flow rate of the fluid. Therefore, by measuring the difference between the upstream and downstream transit times, the flow meter can determine the flow rate of the fluid.
The flow rate V is related to the difference in transit time (T.sub.u -T.sub.d) in the following way: EQU V=(T.sub.u -T.sub.d)D/2T.sub.u T.sub.d cos x,
where D is the distance between the transducers and x is the angle that the sound-propagation path between the transducers forms with the fluid-flow direction.
A common way of measuring transit time is to count the number of cycles of a clock signal between the time at which the sonic pulse is transmitted and the time at which it arrives at the receiving transducer. Of course, the counts for the upstream and downstream measurements must start and stop at corresponding points in the transmitted and received signal pulses if the resulting flow-rate determination is to be accurate. In addition, the detector on the receiver end must not mistake noise for the received signal.
A threshold detector, which senses the received signal's crossing of a preselected threshold, is a conventional apparatus for satisfying these requirements. Generally, the threshold is set high enough that the ambient noise does not trigger the detector, but it is set low enough to detect the received signal. From threshold-crossing time and the signal amplitude, one can calculate the times at which the signal crosses the zero axis, and the actual arrival time of the signal can thus be determined.
When a threshold detector is used, however, the strength of the received signal affects the determination of arrival time. That is, a weak signal may appear to arrive later than a strong signal even though the actual transit times for the signals are identical. There are two reasons for this, both having to do with the shape of the received signal. Typically, the received signal is a series of oscillations that first increase and then decrease in amplitude. When the signal begins with a positive excursion followed by a negative excursion, the detector is generally set to determine the point in time at which the first negative excursion reaches a threshold level. If the signal is too weak, however, the receiver may miss the first negative excursion and detect a negative excursion of a subsequent cycle of the signal. The resulting measurement of transit time is then seriously in error.
Even if the receiver does not miss the first negative excursion, there is another reason why the weaker signal may appear to arrive later than the stronger signal. The slope of the strong signal at a zero crossing is greater than the slope of the weaker signal at its corresponding zero crossing. Thus, even though the two signals arrive at the same time as determined by their zero crossings, the weaker signal reaches the threshold level somewhat later than the stronger signal and thus appears to have a longer transit time. Of course, the error attributable to this factor is significantly smaller than the error attributable to missing the first negative excursion completely. Nevertheless, this error can be significant in some circumstances, such as those in which the distance between the transmitting and receiving transducers is relatively short.
To avoid the errors just described, flow meters generally employ gain-controlled amplifiers to produce a received signal of known amplitude for processing by the threshold detector to generate the flow-rate readings. The degree of amplification is selected to guarantee that the first negative excursion will be detected. And by carefully setting the amplitude of the amplified signal, thereby fixing the slope of the signal at its zero crossings, the actual transit times can be determined with a high degree of accuracy.
Changes in the environment in which the transducers are placed, however, may require recalibrating the flow meter to maintain accurate and reliable operation. For example, repositioning the transducers by moving them farther apart reduces the strength of the received signal. Unless the meter is recalibrated to account for the weaker signal, the resulting flow measurements may be erroneous. As a rule, a flow meter is calibrated for one set of transducers, and any recalibration must be done manually. Recalibration is therefore both time-consuming and inconvenient, and it requires a skilled technician using specialized test apparatus. Thus, conventional flow-meter circuitry cannot easily be used to monitor a number of other transducers in situations in which each set of transducers may be positioned in a different region of the channel or in a different channel.
Changes in the ambient temperature also tend to affect the accuracy of the meter readings, although to a much smaller degree than the changes mentioned above. The circuitry commonly used to provide threshold-level detection tends to be sensitive to temperature changes, so such changes affect the time at which the detector senses the arrival of the received signal Techniques to compensate for such effects are available, but they add unwanted expense to the flow meter.
Another shortcoming of existing flow meters that operate as described above is their inability to operate reliably in the presence of noise. One type of noise is a change in the composition of the stream. If unwanted material, such as bubbles injected by a pump or debris floating down a river, is introduced into a stream being monitored, the strength of the received signal may be attenuated or become erratic. The change in the signal is likely to result in incorrect flow-rate readings and, if large enough, may lead to a complete loss of monitoring ability.
A second type of noise is the ambient acoustical signals that are often present in the channel. In an open channel such as a river, this noise may be generated by a passing motor boat or a turbine generator in a nearby power plant. In a closed channel such as a pipe in a chemical plant, it may be generated by a valve opening or closing or by a pump. Regardless of the source, acoustical noise of sufficient magnitude can completely obstruct the operation of the flow meter by providing a false signal that the meter treats as the received signal
In summary, conventional sonic flow meters have numerous shortcomings that seriously limit their usefulness. The limitations are most acutely felt in hydro-plant applications, in which many flow rates must be monitored, the environment is constantly changing, and noise is common.